Yes, WATERMELONS is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, T 1, E 1, R 1, M 3, E 1, L 1, O 1, N 1, S 1.

Is it a valid word?

WATERMELONS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

an African melon
large oblong or roundish melon with a hard green rind and sweet watery red or occasionally yellowish pulp

Also: watermelon vine, citrullus vulgaris.